Platinum Nanoparticles Encapsulated in MFI Zeolite Crystals by a Two-Step Dry Gel Conversion Method as a Highly Selective Hydrogenation Catalyst

A unique and well-controllable synthesis route to encapsulate metallic nanoparticles in the interior of MFI zeolite crystals has been developed. In the first step, hierarchical micro-mesoporous ZSM-5 zeolite was obtained by alkali treatment, and the platinum was deposited mainly in the pores. Then the precursor was covered with a gel similar in composition to silicalite-1 zeolite, which was structurally converted as whole to the Pt-encapsulated MFI zeolite employing the dry gel conversion method. With this method, the metal species, content, size, and encapsulation in the zeolite are easily controllable. The highly thermally stable Pt nanoparticles encapsulated in MFI zeolites kept their original size after a high-temperature catalytic test for CO oxidation. Because of the size selectivity of the MFI zeolite, the current Pt@MFI catalyst was highly active for hydrogenation of nitrobenzene but inert for hydrogenation of 2,3-dimethylnitrobenzene. Also, the Pt@MFI catalyst is highly selective for the hydrogenation of 4-nitrostyrene, whereas impregnated Pt/ZSM-5 is totally nonselective under the same conditions. The high performance of the Pt nanoparticles encapsulated within MFI crystals should bring about opportunities for new catalytic reactions.